Designers of wireless communication systems, such as cellular and personal communications service (PCS), typically desire to implement a cell-based system exhibiting 360 degrees of wireless communications coverage within a predetermined geographical area of each cell. This omnidirectional communications coverage can be achieved by the use of four ninety degree half power beamwidth (HPBW) azimuth horns positioned at the approximate center of the coverage area for a cell. Each horn antenna is assigned to communications coverage for one of the four ninety degree sectors. Cells for a typical cellular application are positioned on a triangular or square grid spacing to provide maximum coverage of a geographical location while minimizing the possibility of transmission loss as a mobile user moves from one cell to the next adjacent cell.
FIG. 1 is a diagram illustrating a representative example of 360 degree communications coverage for a cell, which is achieved by the overlap of four horn antennas exhibiting ninety degree half power azimuth beamwidths. Sectors beams 102, 104, 106, and 108 overlap at cross-over points 110, 112, 114, and 116, thereby forming overlap regions 118, 120, 122, and 124. A circle (shown by dashed lines) having a diameter D.sub.O connects the cross-over points 110, 112, 114, and 116 and illustrates the useful gain of the 360 degree coverage pattern achieved by the sector horns. Specifically, the gain of a sector horn above the level defined by the diameter D.sub.O represents excess gain that is not useful for a cellular communications application because of possible interference with overlapping the coverage of adjacent cells within the geographical coverage area. For example, excess gain can have a harmful effect on frequency division multiple access (FDMA) and time division multiple access (TDMA) applications because interference can be generated within overlapping neighbor cells using the same frequency band or in use at the same time.
In view of the foregoing, there is a need in the art for azimuth beam shaping for a horn antenna used for cellular communication applications. There is a further need for a sector antenna exhibiting a square "flat-top" beam having a peak gain over a ninety degree field of view, wherein the peak gain is less than the excess gain level exhibited by prior sector horn antennas. A combination of these improved antennas, each covering a ninety degree sector to support the overall coverage objective of 360 degrees, would preferably exhibit higher minimum gain in the desired cell sector area and lower interfering gain in adjacent cells.
Designers of cellular communication systems also rely upon antennas exhibiting a shaped beam in the elevation plane because elevation beam shaping supports the control of front-to-back cellular coverage. For example, the use of an antenna characterized by a narrow elevation beam pattern with sidelobe nulls for a cellular communications application can result in undesirable "holes" or open areas for cellular coverage. In contrast, use of an antenna characterized by a wide beamwidth in the elevation plane for a cellular communications application typically results in a significant reduction of range coverage when compared to the narrow beamwidth antenna. This is a result of a reduction of gain associated with a corresponding increase in elevation beamwidth for the wide beamwidth antenna. To achieve a desired cellular coverage range (or gain) while reducing coverage dropout as a result of elevation pattern nulls, there is a need for an antenna exhibiting an elevation beam pattern having shaped beam with minimal sidelobe nulls.
In summary, there is a need for a cellular communications antenna having adjustable shaping of the beam pattern in the azimuth plane and/or the elevation plane. There is a further need for an antenna characterized by a square "flat-top" beam in the azimuth plane and a peak gain that is consistent over a predetermined field of view. There is a further need for an antenna exhibiting a shaped or "CSC.sup.2 " beam pattern within the elevation plane and minimal sidelobe nulls along the lower pattern edge.